Nonblocking coated sheet material

ABSTRACT

A FLEXIBLE PACKAGING SHEET MATERIAL BEARING A TACKY WAX COMPOSITION COATING AND A NONBLOCKING OVERCOATING OF A POLYAMIDE.

United States Patent O 3,574,669 NONBLOCKING COATED SHEET MATERIALArthur Ruthven Chase, Neenah, Wis., assignor to American Can Company,New York, N.Y. No Drawing. Filed July 22, 1968, Ser. No. 746,241 Int.Cl. B32b 23/08 U.S. Cl. 11776 1 Claim ABSTRACT OF THE DISCLOSURE Aflexible packaging sheet material bearing a tacky wax compositioncoating and a nonblocking overcoating of a polyamide.

BACKGROUND OF THE INVENTION This invention relates to a heat-scalablewrapping material and in particular to such material having anonblocking surface overcoating applied thereto. More specifically, theinvention relates to a wrapping material Including a flexible packagingmaterial substrate such as paper or cellophane coated on one side with athermoplastic heat-sealable wax and elastomer composition which isnormally somewhat tacky at room temperature and finally a nonblockingovercoating composition superposed on the surface of the heat-scalablewax composition.

Flexible packaging sheet materials employing a surface coating of acomposition including a substantial percentage of petroleum waxes ofboth paraffin and microcrystalline types have long been utilized in thepackaging industry because of their excellent combination of protectiveproperties, low cost, freeness from taste and odor, heat sealability andexcellent appearance In terms of gloss and luster. Wax compositionsutilized n many packaging applications include various modify ngadditives such as natural and synthetic rubbers, reslns, polymers andthe like, which alter many of the physical and protective properties ofthe coating in adapting it to specific applications. of particularlnterest in the present invention are wax compositions adapted to themanufacture of coated wrappers for cheese and cheese products, thewrappers bearing coatings comprising petroleum waxes, primarily of themicrocrystalline type, and a small percentage of an elastomer such aspolyisobutylene, generally ranging from 3 to of the total coatingweight. These coatings, when applied to a flexible base sheet such ascellophane, metal foil or combinations of these ma terials with othersheet materials, yield heat-scalable Wrappers having excellentprotective properties for cheese packaging. The surfaces of the coatingsare, however, slightly tacky at room temperature and the wrappers tendto stick together when sheets are stacked one on another or when acoated sheet material is stored in roll form. This sticking or blockingof the coated sheet materials presents a severe problem in the use ofthese wrappers and has necessitated the application to the coatingsurface of a release-treated interleave sheet or a dusting of atack-relieving material. The use of interleaving sheets is inconvenientand expensive and, although many materials, including starch, salt, hardsurfaced waxes, silicones and the like have been applied to coatedcheesewrapper surfaces to relieve blocking, all have had adverse effectson either the heat sealing qualities or the transparency of the wrapper,or have been unacceptable because of transfer to the surface of thecheese when the wrapper is removed from the product. Nonblockingcoatings containing salt crystals also tend to accumulate water on thesurface of the cheese in the vicinity of the salt, giving an undesirableappearance to the product.

It is therefore an object of the present invention to provide animproved cheese packaging material having a nonblocking overcoatingmaterial on the surface of a wax composition coated flexible packagingbase sheet.

It is a further object to provide a flexible, wax composition coatedcheesewrapper having a nonblocking overcoating of improved transparencyand heat sealability, and highly resistant to transfer of any componentof the wrapper to the packaged cheese product when the wrapper isremoved therefrom.

U.S. Pat. 2,833,671 describes a nonblocking coating composition forovercoating wax composition coatings which comprises a normally solid,straight chain fatty acid, a solid, hydrogenated castor oil or solidpolyethylene glycol as a nonblocking agent. The selected material isapplied to the tacky surface of the wax composition in an organicsolvent containing a small percentage of a film-forming binder material,cellulose ethers, polyvinyl acetate and polyamides being cited assatisfactory binders for this purpose. U.S. Pat. 3,340,089 discloses anonblocking overcoating comprising salt crystals as a nonblocking agentsuspended in an organic solvent containing a binding agent such aspolyvinyl acetate, a polyacrylate, a polyamide, shellac or a celluloseether. In each of these patents, the primary consideration is to coverthe surface of the tacky wax composition with a multitude of dis cretesolid particles or granules of material which serve to prevent contactof the tacky coating with a superposed sheet and thus avoid the problemof blocking.

SUMMARY OF THE INVENTION The present invention discloses a nonblockingovercoating composition of improved transparency, which does notinterfere with the heat sealing qualities required of the waxcomposition and which does not transfer to the surface of the packagedproduct, the purpose of the invention being achieved Without thenecessity of a'pplying granules of non-sticky materials to the surfaceof the coated sheet. It has now been found that a completelysatisfactory nonblocking overcoating for tacky wax composition coatedsheets may be achieved by applying on the surface thereof a thin film ofa polyamide dissolved in a suitable organic solvent, followed byevaporative removal of the solvent. A suitable polyamide for thispurpose is Versamid 940, which is a polyamide formed by reaction of adimerized and trimerized fatty acid with ethylene diamine and is sold byGeneral Mills, Inc., under the above trade name. This material isreadily soluble in volatile organic solvents such as isopropyl alcoholand methyl ethyl ketone which wet, but do not dissolve to an appreciableextent the wax composition coatings normally utilized in food packagingat the temperatures at which the overcoating is normally applied.

DETAILED DESCRIPTION OF THE INVENTION To illustrate the principles ofthis invention, a cellophane sheet material coated with a normally tackyWax composition coating comprising microcrystalline wax, 7% petrolatumand 15% polyisobutylene was overcoated with a polyamide, such asVersamid 940, sold by General Mills, Inc., dissolved in isopropylalcohol to form solutions ranging in solids content from 2 to 15%. Allpercentages given above are on a weight basis. The solutions wereapplied to the wax composition surface by means of an etched rollpartially immersed in a bath of the polyamide solution, the excesssolution being doctored from the roll in conventional manner. Thesolvent was removed by evaporation to leave a thin, continuous film ofthe polyamide on the surface of the wax composition. The weight ofpolyamide applied per ream (3,000 sq. ft.) of sheet material wascontrolled by the concentration of the solution applied although it isquite possible also to obtain comparable control by varying the depthand pat- TABLE I Pounds of Overcoating composipolyamide per tion inpercent solids of roam of coated Blocking Heat sealing polyamide sheetresistance quality Excellent. 0.3 cod... Do. 0. 4 Excellcn Do. 0.7 do...Do. 1. 2 do Poor to fair. 2.0 do Poor.

It is evident from the above data that overcoating compositions of thepresent invention when applied to the surface of a tacky, waxcomposition coated sheet material in amounts ranging from about 0.2 lb.to about 1.0 lb. per ream of such coated sheet impart substantiallyimproved blocking resistance while retaining the desired degree of heatscalability. Preferably, the coating is applied in an amount betweenabout 0.3 lb. and about 0.7 lb. per

ream.

In contrast to the above results, ethyl cellulose, when applied in likeamounts as an overcoating, seriously affects the heat sealability of thecoated sheet. Polyvinyl acetate, however, although not deleteriouslyaffecting the heat seal characteristics of the sheet, gave a somewhattacky overcoated surface and was therefore not satisfactory from ablocking prevention standpoint. Among the several materials previouslyutilized as the binding agent component in nonblocking coatingcompositions, polyamides were found to be unique in contributing all ofthe necessary characteristics of such coatings without the addition offurther components such as salt, a fatty acid, starch or other similarmaterials known to assist in blocking prevention.

The polyamide coating, which was homogeneous and continuous, showed notransfer to the surface of cheese vacuum packaged in the overcoatedwrappers described above, the cheese surface remaining completely freeof any foreign material when the packages were opened after six monthsof storage under refrigerator conditions. All packages maintained asatisfactory vacuum for the test period. The protective properties ofthe wax composition coated sheet were unimpaired and the wrappers alsoexhibited low surface friction and superior performance in machinepackaging operations.

The polyamide overcoated sheets are also quite satisfactory for vacuumpackaging of sliced luncheon meats and similar food items, heat sealingcharacteristics, vacuum retention and protective properties having beenam ply demonstrated in six weeks packaging tests with 6 to 8 ounceportions of thin sliced spiced luncheon meat.

In summary, this invention encompasses the discovery that polyamidessoluble in the lower aliphatic alcohols or ketones or similar volatileorganic solvents are uniquely capable of forming a completelysatisfactory non-blocking overcoating on the surface of tacky waxcomposition coated sheet materials without the necessity of anyadditional adhesion-preventing materials. The resulting overcoatedsheets show superior heat sealability, machine packagingcharacteristics, transparency and protective properties.

It is thought that the invention and its attendant advantages will beunderstood from the foregoing description and it will be evident thatmodifications may be made within the spirit of the invention, theembodiments herein described being exemplary rather than limiting.

I claim:

1. A flexible packaging sheet material coated with a heat scalable,normally tacky wax composition and overcoated with between about 0.2 lb.and about 1.0 lb. per ream of a continuous evenly distributed film of anonblocking composition consisting essentially of a polyamide, whereinsaid polyamide is formed by reaction of a dimerized and trimerized fattyacid with ethylene diamine.

References Cited UNITED STATES PATENTS 2,728,737 12/1955 Wittcoif161--235X 2,833,671 5/1958 Funk et al. ll792X 3,340,089 9/1967 Bougie117-76(F) WILLIAM D. MARTIN, Primary Examiner R. HUSACK, AssistantExaminer US. Cl. X.R.

